1. Field of the Invention
The present invention relates to a process for fabricating a projection electron lithography mask and a removable, reusable cover for use therein, and more particularly, to a technique for fabricating a projection electron lithography mask and a removable, reusable cover for use therein, wherein the geometry of the cover is matched to the geometry of an active region (membrane plus strut) of the projection electron lithography mask.
2. Description of the Related Art
Projection electron beam lithography, such as Scattering Angular Limitation Projection Electron Beam Lithography (SCALPEL.TM.), utilizes electron beam radiation projected onto a patterned mask to transfer an image of that pattern into a layer of energy sensitive material formed on a substrate. The image is developed and used in subsequent processing to form devices such as integrated circuits.
The SCALPEL.TM. mask has a membrane of a low atomic number material on which is formed a layer of high atomic number material. The layer of high atomic number material has a pattern delineated therein. Both the low atomic number membrane material and the high atomic number patterned layer of material are transparent to the electrons projected thereon (i.e., electrons with an energy of about 100 keV). However, the low atomic number membrane materials scatters the electrons weakly and at small angles. The high atomic number patterned layer of material scatters the electrons strongly and at large angles. Thus, the electrons transmitted through the high atomic number patterned material have a larger scattering angle than the electrons transmitted through the membrane. This difference in scattering angle provides a contrast between the electrons transmitted through the membrane alone and the electrons transmitted through the layer of patterned material formed on the membrane.
This contrast is exploited to transfer an image of the pattern from the mask and into a layer of energy sensitive material by using a back focal plane filter in the projection optics between the mask and the layer of energy sensitive material. The back focal plane filter has an aperture therein. The weakly scattered electrons are transmitted through the aperture while the strongly scattered electrons are blocked by the back focal plane filter. Thus, the image of the pattern defined in the weakly scattered electrons is transmitted through the aperture and into the layer of energy sensitive material.
FIG. 1 is a schematic diagram illustrating the concept of a conventional SCALPEL.TM. system. A beam B of electrons 10 is directed towards a scattering mask 9 including a thin membrane 11 having a thickness between about 1,000 .ANG. and about 20,000 .ANG. (0.1 .mu.m and about 2 .mu.m thick.) The membrane 11 is composed of a material which is virtually transparent to the electron beam B composed of electrons 10. That is to say that electrons 10 in beam B pass through membrane 11 freely in the absence of any other object providing an obstruction to the path of electrons 10 in the beam B as they pass from the source of the beam through the membrane 11.
Formed on the side of the membrane 11 facing the beam 10, is a pattern of higher atomic number, higher density scattering elements 12 to provide a contrast mechanism that enables reproduction of the mask pattern at the target surface. The scattering elements 12 are patterned in the composite shape which is to be exposed upon a work piece 17 (usually a silicon wafer) which is coated with e-beam sensitive resist, which as shown in FIG. 1 has been processed into pattern elements 18. The electrons 10 from the e-beam B which pass through the mask 9 are shown by beams 14 which pass through electromagnetic lens 15 which focuses the beams 14 through an aperture 16' into an otherwise opaque back focal plane filter 16. The aperture 16' permits only electrons scattered at small angles to pass through to the work piece 17.
A conventional SCALPEL.TM. exposure tool is illustrated in FIG. 2. The exposure tool 20 includes a source 22 (usually an electron gun), a mask stage 24, imaging optics 26, and a wafer stage 28. The mask stage 24 and the wafer stage 28 are mounted to the top and bottom of a block of aluminum, referred to as the metrology plate 30. The metrology plate 30, which is on the order of 3000 lbs., serves as a thermal and mechanical stabilizer for the entire exposure tool 20.
FIG. 3 illustrates the conventional mask stage 24, the imaging optics 26, and the wafer stage 28 in more detail. As illustrated in FIG. 3, the source 22 outputs an electron beam, which is aligned and focused on a lens C1 by a gun alignment deflector 40 and a shaping aperture 42. The electron beam is further focused on a lens C2 by a beam blanking deflector 44, an illumination deflector 46, and blanking aperture 48. After passing through lens C2, the electron beam impinges on the mask 9 and is focused on the wafer 17 utilizing lenses P1 and P2 and deflectors P1 and P2 and a SCALPEL.TM. aperture 50.
The conventional SCALPEL.TM. mask 9 is formed by a process by which the higher atomic number, higher density scattering elements 12 are formed from a polymeric film (or resist) that is spin-coated on the wafer 17 at selected locations. However, during the spin coating process, resist remains on the wafer 17 at undesired locations.
The unwanted regions are primarily located outside the active region (illustrated as element 60 in FIG. 4). Typically, the unwanted resist covers alignment marks (illustrated as element 62 in FIG. 4) which are patterned during the SCALPEL.TM. mask blank metal deposition process. The removal of the resist over the marks improves the ability to detect the marks by subsequent exposure of the mask, inspection and metrology tools. In addition to removing the resist from the marks, grounding pads in each comer of the metalized region are exposed so as to allow for a point of contact for grounding the surface during direct write e-beam exposure. Therefore, a method is required to provide for the removal of the resist from the unwanted regions of the SCALPEL.TM. mask.
It is also desirable to selectively remove the resist from the unwanted regions of the SCALPEL.TM. mask and still maintain (a) cleanliness, (b) reusability, (c) feasibility in a production-type environment, and (d) add no adverse effects to the imaging resist characteristics (for example, sensitivity, damage, etc.).
The conventional method of removing unwanted resist from the unwanted regions of the SCALPEL.TM. mask is to use a solvent in a standard radial-type removal. However, this technique is limited to removing resist radially from the edge of the SCALPEL.TM. mask and hence, the active region of the SCALPEL.TM. mask must be circular in shape.
Furthermore, the standard method of using a solvent to remove the resist from the edge of the wafer 17 in a radial fashion requires an additional step in the resist coat process, namely the step of dispensing a solvent (usually via a syringe or tube) over the wafer's edge. Its primary purpose is to allow for the removal of excess resist buildup near the edge of the wafer 17. For SCALPEL.TM. masks with other than circular active regions (such as the rectangular active region 60 of SCALPEL.TM. mask 9 illustrated in FIG. 4 and 7), it is impossible to uncover alignment marks 62 using the standard method without loss of the resist coating in the active region. FIG. 5 is a photograph showing a loss of resist coverage in an active region on a 100 mm SCALPEL.TM. mask blank, using wet etch resist removal.
The topology of the alignment marks also is a problem in that the alignment marks are in the form of crosses and it is difficult (sometimes impossible) to remove resist trapped in the intersections of the alignment marks utilizing the standard wet solvent removal process.